1. Field of the Invention
The present invention relates to a digital to analog (D/A) converting method and a D/A converter, and more particularly to a D/A converting method and a D/A converter with mismatch and glitch insensitivity.
2. Description of the Prior Art
With reference to FIG. 5, a digital to analog converter (DAC) is mainly comprised of a current source array 200 including a plurality of unit current sources 201 for converting received digital binary-weighted input codes to analog signals. The circuit architecture of a single unit current source 201 is shown in FIG. 6 as an example, mainly having a pair of current switches Msw,p and Msw,n and current sources.
With reference to FIG. 7, when the DAC receives a 3-bit binary-weighted input code B2-B3, seven unit current sources U7-U1 will be controlled by the binary-weighted input code B2-B0. The first digit B2, of the binary-weighted input code controls the unit current sources U7-U4, the second digit B1 controls the unit current sources U3-U2, and the third digit B0 controls the unit current source U1.
In accordance with the conventional digital return-to-zero (DRZ) technique as shown in FIG. 8, three current groups (U7, U6, U5, U4), (U3, U2), (U1) are permanently fixed and respectively correspond to the three digits B2, B1 and B0. For a 3-bit binary-weighted input code, either “100” or “011” can be selected as a mid-code. According to the DRZ method, the binary-weighted input codes and the mid-code are alternately input to the DAC during a signal phase and a return-to-zero (RTZ) phase respectively. As can be seen, for the same binary-weighted input codes encountered again, the unit current source switched by the mid-code is always the same. For example, if the received binary-weighted input code is “101” and the mid-code is “100”, the unit current source “U1” is always selected and switched by the mid-code. In other words, the switching transient induced glitches are correlated with the received binary-weighted input codes. This correlation induced distortion tones will degrade the spurious free dynamic range (SFDR) of the DAC.
To overcome the shortcomings of the transition nonlinearity, the present invention develops a D/A converting method to implement the digital return-to-zero with randomization function, whereby the glitches causes by switching transient can be immunized.